Field effect transistor having a spacer layer with different material and different high frequency characteristics than an electrode supply layer thereon

ABSTRACT

There is disclosed a field effect transistor having a channel layer, an electron supply layer, and a spacer layer formed between the channel layer and the electron supply layer. The spacer layer has a thickness for spatially separating a two-dimensional electron gas from donor ions in the electron supply layer, and for forming the two-dimensional electron gas in the channel layer by the Coulomb force of the donor ions. The spacer layer material has better high frequency characteristics than that of the electron supply layer.

This application is a continuation of application Ser. No. 07/793,020,filed Nov. 15, 1991, which application is entirely incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high speed field effect transistor(FET) primarily used in a microwave band.

2. Related Background Art

An excellent high frequency characteristic is required for asemiconductor material of a FET used in a microwave band. Such amaterial includes an InGaAs semiconductor which exhibits a good electrontransport characteristic. Among others, a high electron mobilitytransistor (HEMT, MODFET) which uses In₀.52 Al₀.48 As and In₀.53 Ga₀.47As which match to InP in terms of lattice as an electron supply layerand a channel layer exhibits an excellent high frequency characteristic.Detail of the HEMT is described in IEEE Electron Device Letters, Vol.11, No. 1, January 1990, pages 56 to 62. An AlGaAs/GaAs-HEMT which usesAlGaAs and GaAs semiconductor materials as the electron supply layer andthe channel layer has been put into practice.

The semiconductor material for the electron supply layer is selected tohave a larger energy gap and a smaller electron affinity than that ofthe semiconductor material for the channel layer. When the channel layeris made of GaAs, AlGaAs is selected as the material for the electronsupply layer, and when the channel layer is made of InGaAs, AlInAs isselected as the material for the electron supply layer. Two-dimensionelectron gas is generated in the channel layer by forming the epitaxialstructure of the HEMT by those materials.

Usually, a gate length of the FET is very short, for example less than 1μm. Thus, the electrons which run therealong have a large energy.Accordingly, when a high voltage is applied to a gate electrode, theelectrons do not stay in the two-dimension channel but they may jumpover an energy barrier which is present on a junction plane of thechannel layer and the electron supply layer and are moved to theelectron supply layer. This phenomenon is explained as follows withreference to an energy band chart of FIG. 1.

FIG. 1A shows an energy band on a hetero-junction plane of theAlGaAs/GaAs-HEMT; two-dimension electron gas shown by hatching isgenerated in the GaAs semiconductor near the junction interface. When ahigh voltage is applied to the gate electrode and a high electric fieldis applied to the electrons, the electrons e⁻ in the GaAs semiconductorjump over the energy barrier which is present on the hetero-junctioninterface and are moved to the AlGaAs semiconductor. The same phenomenontakes place in the AlInAs/InGaAs-HEMT shown in FIG. 1B. When a highelectric field is applied to the two-dimension electron gas generated inthe InGaAs semiconductor and a high energy is imparted to the electronse⁻, the electrons e⁻ jump over the energy barrier on the hetero-junctioninterface and are moved to the AlInAs semiconductor.

The AlGaAs or AlInAs semiconductor material which forms the electronsupply layer has a smaller electron mobility and a lower electronsaturation speed than those of the GaAs or InGaAs semiconductor materialwhich forms the channel layer. As a result, when a portion of electronsmoves from the channel layer to the electron supply layer, the electronmobility of the entire current channel decreases and the electronsaturation speed reduces. As a result, a high frequency characteristicof the FET is deteriorated. The deterioration of the characteristic isalso seen in a transfer conductance g_(m) which is an index to indicatea transfer function of the device. Namely, in a characteristic oftransfer conductance g_(m) vs gate voltage, the transfer conductanceg_(m) materially decreases as the gate voltage increases in a positivedirection.

This problem is pointed out in IEEE Transactions on Electron Devices,Vol. ED-31, No. 1, January 1984 and it is noticed as a technical problemin the high speed FET.

SUMMARY OF THE INVENTION

In the light of the above, it is an object of the present invention toprovide a field effect transistor in which a semiconductor layer havinga thickness to spatially separate two-dimension electron gas from donerions of an electron supply layer and form two-dimension electron gas ina channel layer by a Coulomb force of the doner ions and having a betterhigh frequency characteristic than that of the electron supply layer isformed between the channel layer and the electron supply layer.

In accordance with the present invention, when a high electric field isapplied to the two-dimension electron gas and the energy of theelectrons increases so that the electrons jump out of the channel layer,the electrons are moved to the semiconductor layer of the predeterminedthickness having the good high frequency characteristic, which isadjacent to the channel layer. As a result, even if the high electricfield is applied to the channel of the FET, the transport characteristicof the electrons is not deteriorated and the high frequencycharacteristic of the device is assured.

The present invention is particularly effective when it is applied to abasic transistor structure operated in a microwave band or a millimeterwave band and having a gate length of less than 0.25 μm.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art form this detailed description.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1A-1B shows an energy band chart for explaining a problemencountered in a prior art HEMT,

FIG. 2 shows a sectional view of a structure according to one embodimentof a HEMT of the present invention,

FIG. 3A, 3B and, 3C, 3D shows sectional view in respective steps inmanufacturing the HEMT shown in FIG. 2,

FIG. 4 shows an energy band of the HEMT shown in FIG. 2, and

FIG. 5 shows a graph of a gate voltage V_(g) vs transfer conductanceg_(m) of the HEMT shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a sectional view of a structure of one embodiment of theHEMT of the present invention. A method for manufacturing the HEMT isexplained below with reference to a manufacturing step sectional viewshown in FIG. 3.

A buffer layer 2, a channel layer 3, a spacer layer 4, an electronsupply layer 5 and a contact layer 6 are sequentially and continuouslyepitaxially grown on an InP semiconductor substrate 1 (see FIG. 3A). Theepitaxial growth may be carried out by a molecule beam epitaxy (MBE)method or an organic metal vapor phase epitaxy (OMVPE) method. Thebuffer layer 2 is made of undoped AlInAs and has a thickness of 1 μm,the channel layer 3 is made of undoped InGaAs and has a thickness of 150Å, and the spacer layer 4 is made of undoped InP and has a thickness of30 Å. The electron supply layer 5 is made of n-type AlInAs and has athickness of 400 Å, and the contact layer 6 is made of n-type InGaAs andhas a thickness of 100 Å. The electron supply layer 5 has a donerimpurity concentration of 2×10¹⁸ /cm³, and the contact layer 6 has adoner impurity concentration of 5×10¹⁸ /cm³.

The AlInAs material which forms the buffer layer and the electron supplylayer 5, and the InGaAs material which forms the channel layer 3 and thecontact layer 6 have compositions of Al₀.48 In₀.52 As and In₀.53 Ga₀.47As, respectively, so that the respective layers match to the InPsemiconductor substrate 1 in terms of lattice.

The thickness of the spacer layer 4, 30 Å, is thick enough for theelectrons supplied from the electron supply layer 5 to stay in thechannel layer 3. Namely, it is thick enough for the Coulomb force of thedoner ions in the electron supply layer 5 to be applied to thetwo-dimension electron gas in the channel layer 3. Further, the spacerlayer 4 is thick enough to secure a sufficient spatial distance betweendoner ions of the electron supply layer 5 and the two-dimension electrongas in the channel layer 3. The InP material which forms the spacerlayer 4 has a higher electron saturation speed and a better highfrequency characteristic than those of the AlInAs material which formsthe electron supply layer 5. It has a lower electron mobility and ahigher electron saturation speed than those of the InGaAs material whichforms the channel layer 3.

The semiconductor layers in a transistor formation area are selectivelyetched away by mesa-etching and devices are electrically isolated (seeFIG. 3B). Source and drain electrode patterns are patterned on thecontact layer 8 by a conventional photolithography method. Afterward, anAuGe/Ni metal is vapor-deposited and the patterns are lifted off. Theelectrode metals left after the lift-off are alloyed at 400° C. for oneminute to make ohmic contacts to the contact layer 6 to form a sourceelectrode 7 and a drain electrode 8 (see FIG. 3C).

Then a gate electrode is patterned by an electron beam lithographymethod, and a recess is formed in the gate electrode formation area byusing the pattern as a mask (see FIG. 3D). The depth of the recess iscontrolled such that a predetermined drain current is produced, and aTi/Pt/Au metal is vapor-deposited. After the vapor-deposition, theelectrode pattern is lifted off to form a gate electrode 9. In thismanner, the HEMT having the structure shown in FIG. 2 is formed. In FIG.2, the like elements to those shown in FIG. 3 are designated by the likenumerals.

The energy band of the HEMT having such an epitaxial structure is shownin FIG. 4. The areas of FIG. 4 correspond to the electron supply layer(n-AlInAs) 5, the spacer layer (InP) 4, the channel layer (undopedInGaAs) 3 and the buffer layer (undoped AlInAs) 2, from left to right.An energy gap is formed in the channel layer 3 near the junction of thespacer layer 4 and the channel layer 3, and the two-dimension electrongas shown by hatching is generated in the energy gap. An energy bandwhich supports the accumulation of the two-dimension electron gas ispresent in the spacer layer 4 and a higher energy barrier than theenergy band is formed between the electron supply layer 5 and the spacerlayer 4.

When a high voltage is applied to the gate electrode 9 and a highelectric field is applied to the two-dimension electron gas, theelectrons in the channel bear a high energy. As a result, some of theelectrons in the two-dimension electron gas jump out of the energy gapformed in the channel layer 3. The electrons which "jumped-out" areattracted to the electron supply layer 5 by the Coulomb force of thedoner ions in the electron supply layer 5 but they are caused to stay inthe spacer layer 4 by the energy barrier between the electron supplylayer 5 and the spacer layer 4.

As described above, the InP material which forms the spacer layer 4 hasa better high frequency characteristic and a higher electron saturationspeed than that of the AlInAs material which forms the electron supplylayer 5. Also, the InP material has a lower electron mobility and ahigher electron saturation speed than that of the InGaAs material whichforms the channel layer 3. As a result, even if the electrons jump outof the channel layer 3 when the high electric field is applied to thetwo-dimension electron gas, the electrons which jumped-out travel in thespacer layer 4 having the high electron saturation speed. Accordingly,the high frequency characteristic of the device does not deteriorateunlike the prior art device, even if a high electric field is applied tothe two-dimension electron gas.

A gate voltage vs transfer conductance g_(m) characteristic of the HEMTis shown in FIG. 5, in which an abscissa represents the gate voltageV_(g), an ordinate represents the transfer conductance g_(m), a solidline curve 11 shows a characteristic of the HEMT of the presentembodiment, and a broken line curve 12 shows a characteristic of a priorart HEMT. As seen from FIG. 5, in the characteristic curve 11 for thepresent embodiment, the decrease of the transfer conductance g_(m) issuppressed when the gate voltage V_(g) increases in the positivedirection. On the other hand, in the characteristic curve 12 for theprior art, the decrease of the transfer conductance g_(m) is remarkableas the gate voltage V_(g) increases. In accordance with the HEMT of thepresent embodiment, a high transfer conductance g_(m) is assured over awide range of gate voltage.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

I claim:
 1. A field effect transistor comprising:an electron supplylayer formed by a first material, said first material of said electronsupply layer being doped to have donors therein, a channel layer inwhich a two-dimensional electron gas is generated by Coulomb forcecorresponding to said donors in said electron supply layer, and a spacerlayer formed between said channel layer and said electron supply layer,said spacer layer being formed of a second material for spatiallyseparating said two-dimensional electron gas from said donors in saidelectron supply layer, said electron supply layer and said spacer layerbeing formed of different elements, said second material having a betterhigh frequency characteristic and having a higher electron saturationspeed than that of said first material, and said second material havinga lower electron mobility than that of said channel layer and a higherelectron saturation than that of the material of said channel layer. 2.A field effect transistor according to claim 1 wherein said channellayer is formed of undoped InGaAs, said spacer layer is formed ofundoped InP and said electron supply layer is formed of n-type AlInAs.3. A field effect transistor according to claim 2 wherein saidtransistor also comprises:an InP semiconductor substrate; and a bufferlayer formed on said semiconductor substrate and formed of undopedAlInAs; and wherein said channel layer is formed on said buffer layerand is formed of undoped InGaAs; said spacer layer is formed on saidchannel layer and is formed of undoped InP; and said electron supplylayer is formed on said semiconductor layer and is formed of n-typeAlInAs; and further wherein a contact layer is formed on said electronsupply layer and is formed of n-type InGaAs; at least two ohmicelectrodes are formed on said contact layer; and a gate electrode isformed in a recess formed in said contact layer and said electron supplylayer.
 4. A field effect transistor according to claim 3 wherein saidbuffer layer and said electron supply layer are formed of Al₀.48 In₀.52As, and said channel layer and said contact layer are formed of In₀.53Ga₀.47 As.
 5. A field effect transistor according to claim 1, whereinsaid spacer layer has a thickness of substantially 30 angstrom units. 6.A field effect transistor according to claim 5, wherein said spacerlayer is formed on said channel layer and is formed of undoped InPwhereby said InP material thereof has a higher electron saturation speedthan that of said first material of said electron supply layer, and alower electron mobility and a higher electron saturation speed than thatof a material which forms said channel layer.